Image sensors can capture color images by combining a pixel array of photosensitive pixels with a set of color filters. Each pixel in the pixel array is coupled to a filter that removes all but a particular wavelength—that is, a particular color of light—from the light incident on the image sensor. As a result, each pixel in the pixel image captures light of a single color, and for each pixel the color values for the other colors are interpolated.
Spectral crosstalk is a problem that occurs in devices that filter separate wavelengths, as do color filters that are coupled to a pixel array in a color image sensor when a small portion of the optical power that should have ended up in a particular channel—that is, that should have been output by particular filter—actually ends up in another channel, such as an adjacent channel. In an image sensor with a color filter array, spectral crosstalk is often the result of imperfect color filtering: a blue filter, for example, might let through some red and green light, so that a pixel to which the blue filter is coupled will receive more than just the blue light it is supposed to receive. In a CMOS image sensor, crosstalk causes degradation of spatial resolution, color mixing, and image noise. This can have serious effects on the sensor's signal-to-noise ratio (SNR) and hence on the error rate of the system. In an RGB image sensor, sensitivity came can be gained by using a clear filter in the filter array, but it increases crosstalk.
Various methods have been tried to decrease spectral crosstalk, but all have drawbacks. Some other types of crosstalk can be reduced by process profile control between pixels, but spectral crosstalk is a natural material property that can't be reduced by process. Another way to reduce spectral crosstalk is to use thicker color filters; this can significantly reduce crosstalk, but it also reduces the signal strength and the quantum efficiency (QE) of the pixel. Another way to reduce spectral crosstalk is to increase pigment concentration in the filters, but the material's refractive index will increase and induce higher optical crosstalk.